The forge welding together of metal parts, such as plates, fins to tubes, etc., or edge portions of the same part folded so that the edge portions meet at a weld point as such latter part is advanced longitudinally of the part, e.g. when a metal sheet or strip is folded into a tube and the strip is advanced in the direction of the axis of the tube, using high frequency, electrical currents to heat the portions to be welded together, is well known in the art. See, for example, U.S. Pat. Nos. 2,774,857; 3,037,105 and 4,197,441.
In general, the metal parts, or portions of a metal part, to be welded together are heated at the portions to be joined by the high frequency electrical current to forge welding temperature, which normally is at least 1300.degree. F. and can be 2500.degree. F.-2700.degree. F. for steel but which is below the melting temperature of the metal and when the portions to be joined reach the forge welding temperature, they are pressed together to produce a weld between such portions.
The heating current is caused to flow in opposite directions on the opposing faces of the metal portions to be joined or welded together to take advantage of the "proximity effect" which causes the oppositely flowing currents to be increased at the faces as compared to current flowing more remotely from the faces. High frequency currents are used to take advantage of the well-known "skin effect".
In forge welding, the heating current is caused to flow in the portions to be heated to welding temperature either by contacts engaging the parts or by an induction coil which induces the current in the parts. In annealing, the heating current usually is induced in the parts by an induction coil and the temperature for annealing normally is at least 1300.degree. F. and can be in the range from 1400.degree. F.-1500.degree. F. for steel but is below the melting temperature and forge welding temperature of the metal. In the past, the high frequency current sources used have included vacuum tube oscillators. Due to the magnitude of heating currents involved, i.e. thousands of amperes, the vacuum tubes are large and expensive.
Relatively high power solid state, electron devices have been developed relatively recently which can be used in high frequency, electrical power sources to generate the necessary heating currents without the use of vacuum tubes. Such solid state, high frequency generators have certain advantages over vacuum tube oscillators, such as size, lower operating voltage and better theoretical electrical efficiency. For example, the solid state devices are smaller than the required vacuum tubes so that the overall size of the housing of the source can be smaller. To produce the large currents required, the vacuum tubes must be supplied with high voltage power, e.g. thousands of volts, whereas the solid state devices need electrical power at only hundreds of volts.
In addition, the theoretical maximum electrical efficiency, i.e. ratio of the high frequency power output to the low frequency or direct current power required to produce the high frequency power, is about 65% for a vacuum tube oscillator and about 80% for a solid state, high frequency power generator. Electrical efficiency is important in many cases, e.g. when the electrical power supplied to a plant by a utility company is expensive or when it is desired to increase the welding current magnitude used in an existing welding apparatus for various reasons but the utility lines, transformers, etc. must be modified or replaced to supply the additional current required. Accordingly, by replacing a vacuum tube oscillator, which is only part of the welding apparatus investment, by a solid state, high frequency generator, it is possible not only to improve the electrical efficiency and hence, the cost of welding but also to increase the welding currents without increasing the demand on the utility company equipment.
Accordingly, it is apparent that if a solid state, high frequency generator can be used in place of a high frequency generator using vacuum tubes, several advantages result.
One type of solid state high frequency generator is a direct current to alternating current inverter. See, for example, the article entitled "A Comparison of Load Commutated Inverter System for Induction Heating and Melting Applications" appearing in IEEE Transactions on Power Electronics, Vol. 6, No. 3, July 1991 which discloses a current source, parallel tuned inverter and a voltage source, series tuned inverter system, both of which use thyristors, for an induction melting furnace for the melting of metals. As with all generators, the maximum transfer of power from the generator to a load is obtained when the impedance of the load matches the impedance of the generator.
Additionally, because the load commutates the inverter, or causes the inverter to switch, the load must be a resonant circuit tuned at the frequency of the alternating output current. A solid state, high frequency generator has a low impedance compared to the impedance of a generator using vacuum tubes. Because of the load and vacuum tube generator impedances and the voltages involved, a special step-down transformer usually is required with vacuum tube generators to couple the generator to the load. See, for example, U.S. Pat. No. 2,825,033. A generator "load" is, of course, all electrical equipment connected to the output terminals of the generator. In welding and annealing apparatus of the type described, the load impedance is affected by many factors including the nature, size and shape of the parts being welded or annealed, whether an induction coil or contacts are used, connecting lead lengths and impedance, movement of the parts, etc. As a result, the load impedance is difficult, if not impossible, to predict, and historically, when the generator employs vacuum tubes, variable mutual inductance high frequency transformers or high frequency transformers with multiple primary taps between the load and the oscillator to scale the load impedance to the proper value have been used. Other methods employed have been the addition or removal of tank capacitors or the use of tapped inductors with manually adjustable shorting bars. All approaches but the variable mutual inductance transformer suffer from the fact that the load match can only be adjusted at low or no power output, so that an "on-line" or continually optimizing load match system was not possible with these approaches.
A solid state, high frequency generator of the D.C. to A.C. type is relatively sensitive in frequency generated with load impedance changes as compared to the frequency sensitivity of prior art vacuum tube, high frequency generators. In said article, the high frequency energy producing components are isolated from the load by a fixed ratio step-down transformer and the metal to be heated is substantially stationary. With a solid state, high frequency generator, if the load resonant frequency varies, the frequency of the generator output power varies. The load impedance, which is dependent on frequency, also varies and can change the load impedance and generator impedance relationship. Thus, a change in load impedance can cause mismatches in frequency and impedance, both of which are undesirable. While the same type of tapped or variable mutual inductance transformers used in vacuum tube welders may be used with some, most notably voltage fed, inverters, other inverter types, most notably current fed inverters, do not perform well when connected to a transformer. Tapped transformers suffer from their inability to adjust the load impedance at significant power levels while the generator is operating.
While said article describes some of the problems encountered with a melting furnace, it fails to recognize the problems the heating of a moving part or parts such as with forge welding apparatus or annealing apparatus connected to an inverter and operating frequencies of 3 KHz and higher which are needed for annealing and for forge welding. The article proposes the use of a fixed ratio matching transformer which is unsatisfactory for forge welding and annealing apparatus in which the parts move and does not disclose apparatus for automatically compensating for load variations as the heating of the moving parts is continuously produced.
Therefore, it is not a matter of merely substituting a solid state, high frequency generator for a vacuum tube high frequency generator. Instead, special provisions must be made for the control of the load impedance and frequency.
U.S. Pat. Nos. 2,856,499 and 3,145,285 disclose variable reactors for varying the current in a welding load. At the time of the applications for the patents (Feb. 28, 1957 and Jun. 19, 1963), high frequency generators of a sufficient power output for forge welding purposes and using solid state devices were not available, but vacuum tube high frequency generators were, and the inventors were referring at that time to generators other than solid state, high frequency generators as the high frequency source. Such other generators are not affected by load impedance changes in the same manner as solid state, high frequency generators, and the variable reactors were used merely to provide heating current changes and not to provide impedance matching and to overcome frequency shift problems encountered with solid state, high frequency generators.
As pointed out hereinbefore, the heating currents are of a large magnitude, i.e. thousands of amperes, and any series reactor must be able to conduct such currents without overheating. For these reasons, it is necessary that the series reactor have unique features when the heating currents are large.